Functional expression of “cardiac-type” Nav1.5 sodium channel in canine intracardiac ganglia

Abstract

Background The autonomic nervous system has been implicated in several arrhythmogenic diseases, including long QT syndrome type 3 (LQT3) and Brugada syndrome. Scarce information on the cellular components of the intrinsic cardiac ganglia from higher mammals has limited our understanding of the role of the autonomic nervous system in such diseases. Objectives The purpose of this study was to isolate and characterize the electrophysiologic properties of canine intracardiac neurons. Methods Action potentials (APs) and ionic currents were studied in enzymatically dissociated canine intracardiac neurons under current and voltage clamp conditions. Immunohistochemical and reverse transcription-polymerase chain reaction analysis was performed using freshly isolated intracardiac ganglia. Results APs recorded from intracardiac neurons displayed a tetrodotoxin-resistant (TTX-R) component. TTX-R APs were abolished in the absence of sodium but persisted in the absence of external calcium. Immunohistochemical studies showed the presence of TTX-R sodium channels in these ganglia. Sodium currents were characterized by two components with different affinities for TTX: a tetrodotoxin-sensitive (TTX-S) component and a TTX-R component. TTX-S current inactivation was characteristic of neuronal sodium currents, whereas TTX-R current inactivation time constants were similar to those previously reported for Na_v1.5 channels. TTX sensitivity (IC₅₀ = 1.17 μM) of the TTX-R component was in the range reported for Na_v1.5 channels. Expression of Na_v1.5 channels in intracardiac ganglia was confirmed by PCR analysis and sequencing. Conclusion Our results suggest that canine intracardiac neurons functionally express Na_v1.5 channels. These findings open an exciting new door to our understanding of autonomically modulated arrhythmogenic diseases linked to mutations in Na_v1.5 channels, including Brugada syndrome and LQT3.